1. Field of the Invention
The present invention relates to a hearth roll disposed in a continuous heat treatment furnace and used to convey a steal sheet, and particularly to a sprayed coating thermally-sprayed onto the outer surface of the hearth roll.
2. Description of the Related Art
A hearth roll disposed in a heat treatment furnace for steel sheets is used for a long time in a weakly-oxidizing or reducing atmosphere at 600 to 1,300° C. It has been considered that Fe oxides and iron powder adhering to a steel sheet form an adherent deposit called “build-up” on the surface of the hearth roll. However, one recent problem due to an increase in use of high tensile strength steel and changes in furnace operating conditions etc. is build-up of Mn oxides.
When build-up occurs on a hearth roll, flaws are formed on the steel sheet, causing a reduction in quality. To improve build-up resistance, Patent Literature 1 discloses a hearth roll in which Ti nitride or Ti carbide has been thermally-sprayed onto the surface of the body of the roll.
Patent Literature 2 discloses a hearth roll having a coating of cermet composed of a ceramic material such as MgAl2O4 having low reactivity with manganese oxides and MCrAlY containing limited amounts of Al and Cr to reduce reactivity with the manganese oxides.
To overcome a drawback in Patent Literature 1, Patent Literature 3 discloses a hearth roll including: a cermet coating formed of a refractory metal and TiN particles coated with a metal oxide stable at high temperatures; and a metal bonding layer formed between the cermet coating and the roll.    Patent Literature 1: Japanese Patent Application Laid-Open No. Sho. 63-250449    Patent Literature 2: Japanese Patent Application Laid-open No. Hei. 08-67960    Patent Literature 3; Japanese Patent Application Laid-Open No. Hei. 10-195547
As described above, the conventional build-up is composed mainly of Fe. However, in recent years, because of an increase in use of high tensile strength steel and changes in furnace operating conditions etc., Mn build-up composed mainly of Mn rather than Fe is becoming a serious problem.
TiN and TiC in the hearth roll disclosed in Patent Literature 1 themselves are materials having high wear resistance and build-up resistance. However, since the thermally sprayed coating is oxidized during thermal spraying, the structure of the coating has a large number of pores and is very brittle. Therefore, when in contact with a steel sheet, the coating wears and flakes off, and long-term use of the hearth roll is difficult.
In Patent Literature 3 in which this drawback has been improved, a coating formed of cermet is used, and a bonding layer is provided between the coating and the base material of the roll. This allows improvement in wear resistance and thermal shack resistance of the coating. In addition, TiN is coated with a metal. Therefore, during thermal spraying, oxidation of TiN is prevented, and simultaneously the coating metal is converted to oxide having abradability. It has been expected that these can improve build-up resistance.
However, in practice, although the oxidation of TiN during thermal spraying could be prevented to some extent, the degree of prevention was insufficient, and almost no oxidation of the coating metal occurred because the time of flight of the thermal spray material was too short (on the order of few msec). Therefore, the build-up resistance achieved was not sufficient. In addition, the TiN particles must be coated with a metal using a technique such as plating, PVD, CVD, or mechanical alloying. This results in an increase in cost, causing an economic problem.
In Patent Literature 2, thermal shock resistance and wear resistance were improved when the ratio of MCrAlY was high. However, the build-up resistance achieved by limiting the amounts of Al and Cr was not sufficient.
As described above, with the conventional technologies, the above-described required characteristics cannot be satisfied.
The object of each of the above Patent Literatures is to prevent build-up composed mainly of Fe. However, no consideration is given to the recent problem of build-up composed mainly of Mn.